The process by which the mammalian immune system recognizes and reacts to foreign or alien materials is complex. An important facet of the system is the T cell response, which in part comprises mature T lymphocytes which are positive for either CD4 or CD8 cell surface proteins. T lymphocytes can recognize and interact with other cells via cell surface complexes of peptides and molecules referred to as human leukocyte antigens (“HLAs”) or major histocompatibility complexes (“MHCs”). These peptides are derived from larger molecules which are processed by the cells which also present the HLA/MHC molecule. See Male et al., Advanced Immunology (J. P. Lipincott Company, 1987), especially chapters 6-10. The interaction of T cells and complexes of HLA/peptide is restricted, requiring a specific T cell for a specific complex of an HLA molecule and a peptide. If a specific T cell is not present, there is no T cell response even if its partner complex is present. Similarly, there is no response if the specific complex is absent, but the T cell is present. The mechanisms described above are involved in the immune system's response to foreign materials, in autoimmune pathologies, cellular abnormalities, and in responses to cancer.
The ability of the T cell arm of the tumor immune response to distinguish tumor cells from normal tissues with exquisite specificity, provides the basis for the development of T cell based cancer immunotherapy. This specific recognition is the result of the preferential or exclusive expression of some antigens in tumors as compared to normal tissues. Several categories of antigens with more or less tumor-restricted expression have been identified during the last decade. Most of them correspond to non mutated self-antigens with tissue restricted expression, although tumor-specific mutated antigens have also been identified (Robbins et al., J Exp Med, 1996, 183:1185-1192). Tissue-specific differentiation antigens such as Melan-A or gp100 (Kawakami et al, J Exp Med, 1994, 180:347-352; Coulie, J Exp Med, 1994. 180:35-42) expressed by both normal cells of the melanocytic lineage and malignant melanoma cells, and often spontaneously immunogenic in melanoma patients, have been extensively studied. The group of tumor antigens most relevant for the development of generic cancer vaccines, however, is that of the so-called cancer/testis antigens (CTA) (Scanlan et al. Immunol Rev 2002. 188:22-32), whose gene expression is developmentally regulated, being mostly restricted to gametogenic cells but silent in adult normal cells. Possibly as the result of activation of a common gametogenic protein expression program in cancer cells (Old et al. Cancer Immunity 2001. 1:1). CTA are expressed in variable proportions of tumors of different histological types.
Numerous MHC Class I restricted epitopes recognized by tumor reactive CD8+ T cells and specific for antigens in each of the groups listed above have been identified. Interestingly, spontaneous CD8+ T cell responses directed against several of these epitopes have been detected in cancer patients (Valmori et al, Cancer Res, 2000. 60:4499-4506; Valmori et al., Cancer Res, 2001, 61:501-512). In contrast, the identification of MHC Class II restricted epitopes recognized by tumor antigen specific CD4+ T cells has proven to be more difficult possibly because of the relatively low frequency of the latter and/or to the lack of effective identification methods (Klenerman et al, Nat Rev Immunol, 2002, 2:263-272). Lately, however, because of important technical advances, the identification of CD4+ T cell epitopes derived from tumor antigens including CTA has been reported with increasing frequency (Chaux et al. J Exp Med 1999. 189:767-778; Zeng et al. Proc Natl Acad Sci USA 2001. 98:3964-3969).
Because most nonhematopoietic tumors express MHC Class I but not Class II molecules, it has been assumed that the predominant antitumor T cell mediated effector mechanism in vivo is direct killing of tumor cells by tumor antigen specific CD8+ T lymphocytes (CTL). CTL can indeed directly and efficiently lyse tumor cells resulting sometimes in in vivo regression of large tumor masses. It is, however, becoming increasingly clear that both tumor antigen specific CD8+ and CD4+ T cell responses are important for efficient tumor immune response to occur in vivo (Wang, Trends Immunol. 2001. 22:269-276).
The multiple roles that tumor antigen specific CD4+ T cells can potentially play in mediating antitumor functions are being progressively unveiled. These involve different mechanisms from providing help for both priming and maintenance of tumor antigen specific CD8+ T cells, to activation of B cells for production of tumor antigen specific antibodies, and even including more direct effects in the effector phase of tumor rejection. The identification of CD4+ T cell epitopes toward which spontaneous responses arise in cancer patients is of particular interest as it gives the opportunity to analyze such responses and their underlying molecular mechanisms in vivo. Furthermore, there exist many patients who would not benefit from any therapy which includes helper T cell stimulation via the aforementioned peptides. Accordingly, there is a need for the identification of additional tumor associated antigens which contain epitopes presented by MHC Class II molecules and recognized by CD4+ lymphocytes.